The invention relates to a method and an arrangement for assisting a driver of a vehicle during operation by providing the driver with a desired steering feel.
Different steering equipment for assisting a driver to steer a vehicle are well known in the art. Traditionally the driver controls the direction of the vehicle with the aid of a steering wheel mechanically connected to the road wheels through a steering assembly. However, to assist the driver it is common to use an auxiliary system to generate an additional force, which is applied to the steering assembly of the vehicle. The additional force reduces the effort required by the driver in changing the direction of the road wheels. Said additional force can be generated by different techniques, e.g. by a hydraulic drive or an electric motor.
Traditionally, various hydraulic power assisted steering (HPAS) systems have been used to add a certain amount of assist torque or assist force to the steering assembly of the vehicle. Such hydraulic systems are typically based on an assist characteristic, often called boost-curve. The shape of a boost-curve is generally determined by the design of the valve and the pump in the hydraulic system. It follows that the assist characteristic in a traditional HPAS-system is static, i.e. the relation between the steering effort required from the driver and the assist torque supplied by the HPAS-system is dependent on a predetermined and static boost-curve. The operation of a boost-curve based HPAS-system is generally such that a certain torque applied by the driver to the steering wheel results in a certain assist torque applied by the HPAS-system to the steering assembly of the vehicle, where the assist torque increases as the driver needs to apply more torque to the steering wheel and decreases as the driver needs to apply less torque to the steering wheel. The amount of torque, which the driver needs to apply to the steering wheel, is in turn dependent upon the specific driving scenario, e.g. dependent upon the vehicle speed, the vehicle turning angle etc.
The static or nearly static assist characteristic of an HPAS-system makes it difficult or impossible to find an appropriate balance between the transmission of road disturbances to the driver and the delivery of suitable steering feel to the driver.
If an HPAS-system uses high static assist torque, less steering effort is needed from the driver, which can be advantageous e.g. to reduce the steering effort needed for parking manoeuvres. A high assist torque will also reduce the transmission of road disturbances to the steering wheel when the vehicle moves, which is another advantage. However, while a high assist torque reduces the influence from road disturbances, a high assist torque will also make the vehicle sensitive for steering wheel inputs, especially at highway speed, which is a disadvantage since it reduces the steering feel.
If an HPAS-system uses a low static assist torque, more steering effort is needed from the driver. However, this stabilises vehicle response to manoeuvres at highway speed and consequently increases the steering feel which is an advantage. On the other hand, low assist torque will also increase the transmission of road disturbances to the driver, which is an unwanted effect in the steering wheel and consequently a disadvantage.
In recent years the older HPAS-systems has been replaced by various electric power assisted steering (EPAS) systems to be used in connection with vehicle steering.
For instance, U.S. Pat. No. 6,250,419 shows a steering system of a vehicle, wherein a controller is used to control an electric motor in an EPAS-system. The electric motor is controlled depending on estimated pinion torque, steering column angle, electric motor velocity, desired assist torque, estimated driver torque, and a measure of the steering wheel torque applied by the driver. The final control signal, which is received by the electric motor, is then calculated by adding a contribution from a non-linear boost-curve to the result from the controller. The boost-curve maps the amount of input command to the electric motor versus the estimated driver torque. Depending on the estimated driver torque the boost-curve points out an amount of input command to be supplied to the electric motor. The system does not explicitly decouple the control of the steering wheel torque from attenuation of road disturbances, since the output from the boost-curve is influenced by road disturbances inherent in the estimated driver torque.
U.S. Pat. No. 6,219,604 shows a steering system of an automobile, wherein a controller structure for an entire steering system is described. The controller structure uses a steer-by-wire concept, which means that there is no mechanical connection between the steering wheel and the steering gear assembly. If such a mechanical connection were present in the steering gear assembly it would be responsible for transmission of road disturbances to the driver. When this connection is substituted or simulated by e.g. electrical means, road disturbances can be eliminated. To convey a steering feel to the driver a first electric motor, controlled by an electronic steering device, imposes a controllable resistance torque to the steering wheel. The steering wheel resistance torque can be determined and processed by measuring the current applied to a second electric motor that imposes a steering torque to the steering gear assembly, or by directly measuring the torque that is imposed by said second electrical motor to the steering gear assembly. The steering wheel resistance torque may alternatively be calculated by creating a model, using various vehicle data (e.g. vehicle speed, friction between road surface and tires, steering wheel angle, etc).
The prior art references cited above do not specifically address the problem of controlling the steering characteristics experienced by the driver and at the same time controlling the amount of road disturbances transmitted to the driver. Especially, the references do not address the problem of accomplishing a control of the steering characteristics independent from the control of said transmission of road disturbances. In particular they do not address said problems in connection with a vehicle steering assembly comprising a steering shaft arrangement, which mechanically connects the steering wheel to the road wheels.
It is desirable to provide an improved method and arrangement for solving the above problems.
According to a preferred embodiment, the invention relates, according to an aspect thereof, to a method for assisting a driver of a vehicle during operation by providing the driver with a desired steering feel, wherein the vehicle comprises a steering arrangement comprising an input shaft, such as a steering column, connected to a manually operated steering device, an output shaft connected to at least one pair of ground engaging members, or wheels, and a mechanical interconnection connecting the output shaft and the input shaft. The mechanical interconnection is a force transmitting element for transmitting a torque between the manually operated steering device and the ground engaging members.
The ground engaging members, or wheels, can be controlled by a servo or power steering arrangement located between the output shaft and the at least one pair of ground engaging members. The power steering arrangement is used to assist the driver when turning the engaging members. Power steering arrangements are usually electrically or hydraulically operated and are well known in the art.
The method according to an aspect of the invention involves the steps of                detecting a first steering angle for the output shaft, which first steering angle indicates the position of the ground engaging members;        detecting a second steering angle for the input shaft, which second steering angle indicates the position of the steering device;        comparing the first and second steering angles and determining a current state of the steering arrangement; and        controlling a first actuator connected to the output shaft and a second actuator connected to the input shaft in response to the detected first and second steering angles, in order to achieve a desired state of the steering arrangement;        applying a torque to the steering device proportional to the angular difference between the output shaft and the input shaft using a resilient portion in the mechanical interconnection, in order to provide the driver with a desired steering feel.        
The resilient portion is a part of the mechanical interconnection connecting the output shaft and the input shaft. This connecting portion can be made from a suitable resilient material, such as natural or synthetic rubber. Depending on the application the connecting portion can have a maximum angle of twist in the range 5-20°. The maximum angle of twist is the angle above which the angular difference will no longer increase, even if the torque applied over the resilient portion is increased. The first actuator connected to the output shaft and a second actuator connected to the input shaft can be controlled independently, in the same or in opposite directions. When the input and output shafts are rotated at different rates in the same direction, or in opposite directions by the actuators then an angular displacement between the ends of the shafts connected to the mechanical interconnection will be taken up by the resilient portion.
The subsequent text comprises a number of examples where a current state of the steering arrangement requires rotation of the input shaft and/or the output shaft in opposite directions. According to these examples, the method involves the step of controlling the first and the second actuators by applying torque to the input shaft and the output shaft in opposite directions in response to the determined current state of the steering arrangement.
In a first example the method involves the steps of:                detecting a changing first steering angle indicating a deflection of the ground engaging members;        detecting a constant second steering angle indicating a fixed position of the steering device;        controlling the first and the second actuators in response to the detected first steering angle;        counteracting the torque transmitted from the ground engaging members by applying an equal and opposite torque to the output shaft; and        counteracting the torque transmitted from the output shaft by applying an equal and opposite torque to the input shaft to maintain a constant second steering angle for the steering device.        
By controlling the first and the second actuators to apply a fluctuating torque in equal and opposite directions, it is possible to counteract a fluctuating torque transmitted from the ground engaging members during a “bump steer” or “roll steer” condition.
A bump steer condition is a transient state which occurs when at least one steerable wheel encounters an obstacle causing both wheels rise together. A roll steer condition is a transient state which occurs when at least one steerable wheel rises, or when one rises as the other falls. The effect of bump-steer is for the wheel to cause the suspension to move up or down, which in turn can induce movement of the steering linkage connecting the pitman arm of the steering gear to the steered wheel. Bump steer is undesirable because the suspension is steering the vehicle instead of the driver.
By controlling the first and the second actuators to apply a continuous torque in equal and opposite directions to counteract a static suspension induced torque from a steering linkage connecting the ground engaging members and the output shaft. By applying said continuous torque it is possible to maintain the steering device in a predetermined position, in order to provide steering error compensation. Steering error compensation can be used for eliminating a static steering error, causing the steering wheel to be angled off centre. This condition can occur when, for instance, the vehicle is loaded so that the front of the chassis is elevated above a predetermined level. The relative position of the wheel and the steering gear will generate a steering error that cannot be corrected by the steering geometry of the steering linkage. Even if the static steering error is relatively small, such as a few degrees, it is noticeable to the driver when the vehicle is travelling straight ahead.
In a second example the method involves the steps of:                detecting a current first steering angle indicating a deflection of the ground engaging members;        detecting a current second steering angle indicating a steering input from the steering device;        controlling the first and the second actuators in response to the detected first steering angle;        applying a first torque to the output shaft to transmit a steering torque to the ground engaging members in response to the steering input; and        applying a second torque to the input shaft in the opposite direction providing a resistance to the steering input.        
By applying a first torque that is greater than the second torque it is possible to provide a steering feel indicating understeer. In this case the first torque to the output shaft for turning the ground engaging elements is greater than the second torque to the input shaft, wherein the elastic steering force transmitting element provides a resistance to the driver input. This function is desirable when a vehicle with a heavy load is negotiating a bend in the road, wherein the transverse acceleration is above a predetermined value. In order to achieve the required wheel angle to follow the bend, the driver has to turn the steering wheel further than under normal conditions. This will give the driver the impression that the vehicle is understeered, which encourages the driver to reduce speed through bends.
By applying a first torque that is less than the second torque it is possible to provide a steering feel indicating oversteer. In this case the first torque to the output shaft for turning the ground engaging elements is less than the second torque to the input shaft, wherein the elastic steering force transmitting element provides assistance to the driver input. This function is desirable when a vehicle is being manoeuvred at relatively low speed, wherein the transverse acceleration is below a predetermined value. When negotiating tight corners during manoeuvring the vehicle at low speed the driver needs to turn the steering wheel less than under normal conditions to achieve a desired wheel angle. This provides an oversteered response, giving the driver the impression that the vehicle is easily manoeuvred.
The decision to apply the method according to the second example is dependent on the current state of the steering arrangement. The current state of the steering arrangement is in turn dependent on the current loading on said arrangement, which is proportional to the lateral acceleration of the vehicle. The lateral acceleration is determined by conventional means, for instance, by available sensors on the vehicle, such as an acceleration sensor or a vehicle speed sensor in combination with vehicle chassis data (vehicle wheel base, etc.). Means for the determination of the transverse acceleration is commonly available on vehicles and is not part of the invention per se.
The invention further relates, according to an aspect thereof, to a steering arrangement in a vehicle. The steering arrangement comprises an input shaft, or steering column, connected to a manually operated steering device, such as a steering wheel, an output shaft connected to at least one pair of ground engaging members such as wheels, via a steering linkage. The steering arrangement as a mechanical interconnection between the input shaft and the output shaft, and angular sensors arranged to detect the angular positions of the steering device and the ground engaging members. An electronic control unit is arranged to determine a current state of the steering arrangement
The steering arrangement further comprises a first actuator connected to the output shaft, and a second actuator connected to the input shaft. The first and second actuators can be electric or hydraulic motors. An elastic steering force transmitting element connects the input shaft and the output shaft. This connecting portion between the input shaft and the output shaft can be made from a suitable resilient material, such as natural or synthetic rubber.
The first and the second actuators are individually controllable and are arranged to rotate the output shaft and the input shaft, respectively, either in the same or in opposite directions in response to the detected angular positions of the steering device and the ground engaging members, and the current state of the steering arrangement. The steering force transmitting element is arranged to apply a torque to the steering device proportional to the angular difference between the output shaft and the input shaft.
In general, the first and the second actuators can be controlled to act in the same direction in order to provide servo assistance for turning the ground engaging members, either instead of or in conjunction with an existing servo mechanism or steering gear. Alternatively, the first and the second actuators can be controlled to act in opposite directions in order to provide an enhanced steering feel and to assist the driver under predetermined conditions, for instance when unexpected or undesirable feedback is transmitted towards the steering wheel from the ground engaging members or from the suspension via the steering linkage.
According to one example the first and the second actuators are arranged to apply torque to the output shaft and the input shaft in opposite directions in response to the determined current state of the steering arrangement.
In this first example, the first and the second actuators can be arranged to apply a fluctuating torque in equal and opposite directions to counteract a fluctuating torque from the ground engaging members during a bump steer condition.
Alternatively, the first and the second actuators are arranged to apply a continuous torque in equal and opposite directions to counteract a suspension induced torque from a steering linkage connecting the ground engaging members and the output shaft, in order to maintain the steering device in a predetermined position.
According to a second example the first actuator is arranged to apply a first torque to the output shaft to transmit a steering torque to the ground engaging members in response to the steering input; and that the second actuator is arranged to apply a second torque to the input shaft in the opposite direction providing a resistance to the steering input.
In this second example, the first torque is less than the second torque to provide a steering feel indicating understeer. This function is desirable when a vehicle with a heavy load is negotiating a bend in the road, wherein the transverse acceleration is above a predetermined value. In order to achieve the required wheel angle to follow the bend, the driver has to turn the steering wheel further and/or with more effort than under normal conditions. This will give the driver the impression that the vehicle is understeered, which encourages the driver to reduce speed through bends.
Alternatively, the first torque is greater than the second torque to provide a steering feel indicating oversteer. This function is desirable when a vehicle is being manoeuvred at relatively low speed, wherein the transverse acceleration is below a predetermined value. When negotiating tight corners during manoeuvring the vehicle at low speed the driver needs to turn the steering wheel less and/or with less effort than under normal conditions to achieve a desired wheel angle. This provides an oversteered response, giving the driver the impression that the vehicle is easily in all manoeuvred.
As stated above, the decision to apply the method according to the second example is dependent on the current state of the steering arrangement. The current state of the steering arrangement is in turn dependent on the current loading on said arrangement, which is proportional to the lateral acceleration of the vehicle. The lateral acceleration is determined by the electronic control unit using available sensors on the vehicle, such as an acceleration sensor or a vehicle speed sensor in combination with known vehicle chassis data (vehicle wheel base, etc.).
In addition, the first and the second actuators can be arranged to apply torque to the output shaft and the input shaft in the same direction in response to the determined current state of the steering arrangement, in order to provide steering assistance.
The invention further relates to a vehicle comprising a steering arrangement as described above.
The present invention also relates to a computer program, computer program product and a storage medium for a computer all to be used with a computer for executing the method as described in any one of the above examples.
The invention relates to a computer program comprising program code means for performing all the steps of the method described above when said program is run on a computer.
The invention also relates to a computer program product comprising program code means stored on a computer readable medium for performing all steps of the method described above when said program product is run on a computer.
The invention further relates to a storage medium such as a computer memory or a nonvolatile data storage medium for use in a computing environment, the memory comprising a computer readable program code to perform the method described above.